T.38 vs G.711

    Businesses migrating to VoIP find it desirable in order to move their fax traffic onto to the Fax over IP(FoIP). In this document, I’ll try to share with readers the performance two principal options for sending fax: T.38 fax relay and G711 fax pass-through. Except this methods, there are many fax settings such as fax speed, redundancy and error corection, can have significant impact on FoIP performance.

In first, we should know T.38 protocol as sender just fax over data network. You know that voice communication can not be carried over data network in the same way and T.38 must be known like this.

Before, fax was designed with analog signal and this is best reason why it can not travel over digital voip network. Engineers try to find ways for converting analog signal of fax to digital signal so it could be sent over voip with old fax devices. 

If you want to use your old fax device, you need to connect your voip network. Some voice gateways and ATA help to convert analog signal to digital signal (two way) for you, but there is a condidation about gateways. It should be capable for T38 protocol.  In addition, fax is converted to image through T38 and it will be converted again to fax, when it arrived. 

There is one disadvantage of T.38 that gateway support fro fax parameters like V.34 transmission speed and error correction mode (ECM) is not universal. Also, in mixed network environment of packet-based and circuit-switched connections, T.38 has transcoding overhead. It caused latency and cost.

G.711 is offered from ITU-T for Pulse Code Modulation of voice frequencies.  It requires high bandwidth approximate 64 kbps, because it uses uncompress method. The digital fax data is converted to a PCM audio stream and then sent as G.711 Real-time Transport Protocol (RTP) packets.

In addition, G.711 is not convenient for tranporting fax over IP network, because It does not support packet redundancy. I think that the best positive different between two methods is redundancy.

Consuquently, when you use G.711 for transporting audio, it can miss some part of audio. That is not known significant issue from customers, because it can be filled gaps of audio by human listener. But when used to transmit modem data, any loss is very important for users and they can not get back lost data.

Lets check call flow SIP and T.38 - G.711;

This call flow is shown below are between two sip users and the call starts with an audio media sssion then switched to over T.38.

After the seconf ‘ack’, second sip user sends re-invite to first user. When you check traces of re-invite packet by whireshark, you can see T.38 protocol in the SDP (Session Description Protocol). Part of between two blue line defines fax transmission between two sip users.

When G.711 is used for fax pass-through method between two sip users, you will find PCM instead of T.38 SDP. After the began with an audio media session, switches over to an audio connection.

How Detect Fax Call in VoIP

Fax machines work up to 14400 bps. A fax call starts with a 1100-Hz calling (CNG) tone from the originator and a 2100-Hz ANS acknowledgment tone from the responder. These tones are used to detect the fax call in VoIP. 

The V.21 preamble is generated immediately after the ANS tone from the responding fax machine. The V.21 preamble indication is also used in addition to the CNG/ANS tones for detecting the fax call. The tones are indicated in the beginning of the call. SG3 fax machines use a V.34 modem for high speed that generates an answering tone consisting of amplitude and 180 ° phase reversal modulation (/ANSam). In this case, the fax detector has to support the detection of an /ANSam detection indication to switch to a fax call.

Please do not hesitate to ask any question about T.38 and G.711 with using my e-mail.